Gilles Duranton and Matthew A. Turner investigate the relationship between interstate highways and highway vehicle kilometers traveled in U.S. cities.
We investigate the effect of lane kilometers of roads on vehicle-kilometers traveled (vkt) for different types of roads in us cities. For interstate highways in the densest parts of metropolitan areas we find that vkt increases in exact proportion to highways, confirming the 'fundamental law of highway congestion' suggested by Downs (1962, 1992). This relationship also approximately holds for other important roads in dense areas and for interstate highways in less dense parts of metropolitan areas. These findings and others in the paper imply something broader than Down's law, a law of road congestion that applies to highways and major urban roads in metropolitan areas. In turn, this suggests that increased provision of highways and major urban roads is unlikely to relieve congestion of these roads.
Our investigation is of interest for three reasons. First, an average American household spent 161 person-minutes per day in a passenger vehicle in 2001. These minutes allowed 134 person-km of auto travel at an average speed of 44 km/h. Comparison with corresponding data from 1995 show that the time spent on routine household travel increased by 10% in only six years, while distances remained constant. Multiplying by the number of households in the us and any reasonable dollar value of time, we see that society allocated billions of dollars more to traffic congestion in 2001 than in 1995. That Americans rank commuting among their least enjoyable activities (Krueger, Kahneman, Schwarz, and Stone, 2008) confirms our suspicion that the costs of congestion are large. To the extent that these resources could have been better allocated, understanding congestion and the effect of potential policy interventions is an important economic problem.